A synchronous, clocked communication system having characteristics of equidistance is understood to be a system of at least two stations which are connected to one another via a data network for the purpose of the mutual exchange of data or, the mutual transmission of data. In this arrangement, the data exchange is cyclic in equidistant communication cycles which are predetermined by the communication clock used by the system. Stations are, for example: centralized programmable controllers; programming, configuration or operating devices; peripheral devices such as e.g., input/output modules, drives, actuators, sensors, stored-program controllers (SPS) or other control units, computers or machines which exchange electronic data with other machines, particularly those which process data of other machines. In the text which follows, the control units are understood to be closed-loop or open-loop control units of any type. The data networks used are, for example, bus systems such as, field bus, professional field bus, Ethernet, industrial Ethernet, FireWire or internal PC bus systems (PCI) etc.
In distributed automation systems, for example in the field of drive engineering, clocked data networks or bus systems are often used. Some of the connected stations are used as master devices, e.g. control units such as numeric or stored-program controllers or configuration devices, and other stations are used as slave devices such as drives or peripheral devices. Automation components of both categories can operate in a clocked manner, i.e., these stations can synchronize to a communication clock used in the data network. This means that the communication clock can be taken over by the stations via the data network used and certain processes can be controlled in synchronism with this communication clock. According to IEC 61491, EN61491 SERCOS interface—Technische Kurzbeschreibung (basic technical description) (http://www.sercos.de/deutsch/doku_freier_bereich.htm), this is currently used and carried out in distributed automation systems. However, it is not possible at present to introduce a relative clock which is valid for such a system, and to the respective valid relative time of which all stations connected to the communication system can be synchronized. As a result, for example, applicative event sequences cannot be specified and carried out, or at least not with sufficiently great accuracy in time.